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Abstract

Context. Spectral line profiles of several molecules observed towards the pre-stellar core L1544 appear double-peaked. For abundant molecular species this line morphology has been linked to self-absorption. However, the physical process behind the double-peaked morphology for less abundant species is still under debate.
Aims. In order to understand the cause behind the double-peaked spectra of optically thin transitions and their link to the physical structure of pre-stellar cores, we present high-sensitivity and high spectral resolution HC¹⁷O⁺ J =1−0 observations towards the dust peak in L1544.
Methods. We observed the HC¹⁷O⁺(1−0) spectrum with the Institut de Radioastronomie Millimétrique (IRAM) 30 m telescope. By using state-of-the-art collisional rate coefficients, a physical model for the core and the fractional abundance profile of HC¹⁷O⁺, the hyperfine structure of this molecular ion is modelled for the first time with the radiative transfer code loc applied to the predicted chemical structure of a contracting pre-stellar core. We applied the same analysis to the chemically related C¹⁷O molecule.
Results. The observed HC¹⁷O⁺(1−0) and C¹⁷O(1−0) lines were successfully reproduced with a non-local thermal equilibrium (LTE) radiative transfer model applied to chemical model predictions for a contracting pre-stellar core. An upscaled velocity profile (by 30%) is needed to reproduce the HC¹⁷O⁺(1−0) observations.
Conclusions. The double peaks observed in the HC¹⁷O⁺(1−0) hyperfine components are due to the contraction motions at densities close to the critical density of the transition (~10⁵ cm⁻³) and to the decreasing HCO⁺ fractional abundance towards the centre.